The present invention relates generally to the material handling industry and more particularly, this invention pertains to the overhead material handling industry using applications involving dual hoists.
Within the overhead material handling industry, applications involving dual hoists can be inefficient, costly to implement and wrought with safety concerns. Before the use of Programmable Logic Controllers (PLCs), dual trolley loads were raised utilizing two separate motor and drive packages. Since the hoists operated independently, the loads often would rise at incongruent speeds, causing an un-even lift and potentially unsafe working conditions.
Until recently, the only remedy for this situation was to use a PLC in conjunction with the motor and drive packages. Two drives would be applied to two separate motors and encoders, giving hook position feedback to a PLC. The PLC would control the drives in order to synchronize the speeds of each hook. Though it accomplished the mission of synchronizing the hook speeds, it also increased the complexity and cost of the operating system.
Current products and techniques tend to be either open loop or require an extra sensor of some sort. Open loop products give a simultaneous run command and expect the two hoists to follow the same command well enough to perform a synchronized lift. Other devices require a load cell or some other tension/torque measurement device to detect loading of individual cables and adjust speed on drives based on load. One final method is to monitor position from each motor in an external device, such as a PLC, and then adjust the speed command to individual drives based on the position feedback from their respective motor and encoder.
Several United States Patents have been issued for alternative technologies. These include U.S. Pat. No. 4,266,175, issued to Braun et al. on May 5, 1981; U.S. Pat. No. 4,665,696, issued to Rosman on May 19, 1987; U.S. Pat. No. 5,210,473, issued to Backstrand on May 11, 1993; U.S. Pat. No. 5,324,007, issued to Freneix on Jun. 28, 1994; U.S. Pat. No. 5,625,262, issued to Lapota on Apr. 29, 1997; U.S. Pat. No. 5,874,813, issued to Bode et al. on Feb. 23, 1999; and U.S. Pat. No. 6,047,581, issued to Everlove, Jr. et al. on Apr. 11, 2000.
U.S. Pat. No. 4,266,175 issued to Braun, et al. on May 5, 1981 discloses a method for thyristor control of AC wound rotor motors. This patent involves controlling the switching devices which generate the variable frequency output voltage to a motor.
U.S. Pat. No. 4,665,696 issued to Rosman on May 19, 1987 discloses a hydraulically operated hoist for containerized freight or the like. As may be noted in the claims section, this patent refers specifically to a lift system that is hydraulically actuated. Additionally, per Column 10, lines 30–39 and FIG. 5, the ability to help level the load is produced through a level-sensitive transducer. This transducer, in turn, causes the hydraulic pressure to adjust the load to be leveled.
U.S. Pat. No. 5,210,473 issued to Backstrand on May 11, 1993 discloses a system with delay timer for motor load equalization. This patent is directed to a circuit utilizing a control circuit providing a motor speed signal. Two separate motor connected inverters monitor the signal and generate command ramps for the motor speed control. Each inverter includes a microprocessor means which repetitively runs through its program to scan a sequence of program instructions. One of the items read is the motor speed signal which is utilized to control the speed of the motor. The essential purpose of this device is to attempt to provide a more uniform reference to both motor drives. These motor drives are run asynchronously with each motor following the commands of their respective drives. By setting internal parameters related to acceleration, deceleration, or other pertinent speed control parameters, a similar path will be followed. This device attempts to allow each motor and drive to proceed through initial start-up conditions, such as receiving a run command, generating initial torque, and opening the brake, and then wait at some speed for a set dwell time to ensure both motors are ready to run at the commanded reference speed. At this point the motors begin to follow the independent command trajectories generated by their respective drives.
U.S. Pat. No. 5,324,007 issued to Freneix on Jun. 28, 1994 discloses a load-hoisting system having two synchronously rotating drums operating in parallel. This system has a single motor and controller driving two output shafts. This patent is for a system that is mechanically redundant in order to prevent a load from falling due to a single mechanical failure.
U.S. Pat. No. 5,579,931 issued to Zuehlke, et al. on Dec. 3, 1996 disclosing a system for a lift crane with synchronous rope operation. This method is used by a lift crane which uses two separate ropes attached to a single hook in such a manner that tension can be measured between the two ropes. If the tension changes such that it indicates one of the ropes is moving faster than the other, the speed can then be adjusted so that the two ropes lift at the same speed.
U.S. Pat. No. 5,625,262 issued to Lapota on Apr. 29, 1997 discloses a system for equalizing the load of a plurality of motors. This patent details a method of load sharing between two drives utilized in tandem to control a single hoist. This is accomplished by issuing a torque reference command from the first inverter to the second inverter as noted in column 3, lines 3–16. In column 3, lines 17–30 of this patent, it is claimed that a speed indication of the first motor is sent to the second motor to assist in controlling the speed of the second motor. The only connection between the two drives that is necessary and/or discussed is line 150 of FIG. 3 as referenced in column 7, lines 31–34. This is the torque reference generated by the first drive, labeled 96, and sent to the second drive, labeled 94. Column 4, lines 59–64, reference controller operating by a lever to provide input signals to the drives producing a speed command for the drives. This is one of two common methods of generating a speed command to a drive. This allows for an analog command signal with a range of speed commands from the minimum programmed speed up to the maximum programmed speed. The second method typically uses pushbuttons, but could be any type of discrete input, to generate discrete speed input commands corresponding to pre-programmed levels. This is common practice in the crane and hoist industry.
U.S. Pat. No. 5,874,813 issued to Bode et al. on Feb. 23, 1999 discloses a control method, especially for load balancing of a plurality of electromotor drives. As noted in the background of this patent it is known in the art to utilize a control process in which the difference between the armature currents of two successive drives produces a signal which is used to reduce the speed setpoint in the speed control circuit of the more strongly loaded drive to bring about a load balancing. As noted in Column 4 each of the electric motors have a separate speed control circuit which comprises a speed controller and a proportional feedback unit connected in parallel to the controller. As noted in Column 5, beginning at Line 4, the output of the speed controller is feed into an adder so that the setpoint value can be corrected and delivered to the current controller. The primary purpose of this controller is to provide the proper torque or tension throughout a system in which material is pulled through or across multiple points by multiple motors. In this type of application, controlling the tension is typically the most desired feature of a control system. This explains the primary concentration on controlling current, as torque is directly proportional to current. As stated in column 3, lines 26–30, the effect of the speed feedback controller is limited to allow the separate load-balancing controller to dominate performance in this system.
U.S. Pat. No. 6,047,581 issued to Everlove Jr., et al. on Apr. 11, 2000 discloses a drive system for vertical rack spline-forming machine. This patent discloses the use of two or more motors for driving a spline-forming machine. This invention utilizes a PLC to provide output to two circuit motor power control modules which advance the slide. As noted by the description in this patent a home position is utilized to synchronize the position of the two motors. In the machine tool industry, it is common practice to synchronize a mechanical component which requires dual (multiple) drives such as these rails on a slide by using some sort of electronic home position and an external controller then to keep the two (or more) servomotors running synchronously.
Current control methods typically utilize one of the following methods for synchronizing multiple hoists:
Mechanical coupling between the hoist drums combined with load sharing between the motor drives.
An external sensor to detect differences in speed, alignment or loading of hooks and use of the information to align the hooks.
An external controller used to receive a speed reference and an encoder feedback from each motor drive and use this information to provide the appropriate reference to each drive to maintain alignment of the hooks.
What is needed then is a simplified construction and system for a Multiple Hoist Synchronization Apparatus and Method.